NIH Human Stem Cell Facility is a group at the Bethesda campus that is focused on characterizing the conditions for growing human embryonic stem cells. Because hES cells give access to early developmental events and can generate many terminally differentiated cell types, they may form the basis of a powerful new technology. Despite this promise, technical limitations may blunt the wide use of human ES cells. The most fundamental concern would be caused by genetic instability. The NIH-HSCF has routinely grown hES cells from August 2003. In the following year, most of the hES cells on the NIH registry were obtained, expanded in culture and analyzed by karyotype and FACS analysis. In the period to 7/2004, the central achievement of NIH-HSCF was to show that sub-clones of a hES cell line had a stable genome using a high-resolution genome scanning method. This result provides the most convincing evidence to date that at least one hES cell line can be grown for long periods without genetic change. In the reporting period to 7/05, the major focus was to develop general standards to assess the growth state of hES cell lines in an international collaboration organized through the International Stem Cell Consortium. The NIH-HSCF was one of the few sites contributing multiple hES cells from different providers grown under standard conditions. In the most recent reporting period to 7/06, the major achievements are (1) to establish conditions for homolgous recombination in hES cells, (2) to develop molecular tests for stress responses in the undifferentiated state and (3) to assist Dr. Thomas Cimato (NHLBI) in the isolation of human precursors for endothelial and blood cells.